There are about 500,000 pieces of space debris as small as 1 cm in Earth's orbit, and about 23,000 trackable objects larger than 10 cm. Dead rockets fall into an interesting and dangerous category. The 956 known rocket bodies in space make up just 4% of trackable objects, but account for nearly a third of the total mass. The largest empty rockets, most of which were discarded by Russia in the 1980s, 1990s and 2000s, weigh up to 9 tons. That's as much weight as an elephant.
The abandoned upper stage, the upper part of the rocket that lifts a satellite or spacecraft to its final orbit, will drift around Earth once launch is complete. They are uncontrollable, spinning haphazardly, and pose great danger. If the two collided, they would create a deadly cloud of up to 10,000 to 20,000 pieces of debris, says Darren McKnight, a space debris expert at U.S. debris tracking company LeoLabs.
Such an incident can happen at any time. “I expect there will be a collision involving them at some point,” said Hugh Lewis, a space debris expert at the University of Southampton in England. “There’s too much stuff out there.” This could cause major problems, rendering parts of Earth's orbit unusable or, in the worst case, leading to a runaway chain reaction of collisions known as Kessler syndrome. This could render some orbits unusable or make human spaceflight too risky until debris falls back into the atmosphere decades or centuries later.
The number of rockets abandoned in orbit has fallen since the United Nations introduced new guidelines in 2007 requiring objects to be removed from space within 25 years of their operational life. Most upper stages now retain some fuel so they can be pushed back into the atmosphere after launch. “Now they tend to reserve some propellant to help with deorbiting,” says Lewis. But thousands of “legacy objects” remain from before the rule was introduced, Lewis adds.
The rocket that JAXA is targeting as part of its Commercial Removal of Debris Demonstration (CRD2) program is the upper stage of Japan's H-IIA rocket, which launched a climate satellite in 2009. It weighs 3 metric tons and is as big as a bus. It orbits our planet at an altitude of 600 kilometers (373 miles). If not managed, it will remain in orbit for decades before Earth can pull it into the atmosphere, Lewis says. At that point it will burn up, and any remaining debris will likely fall into the ocean.
ADRAS-J's mission is to figure out how to get it back into the atmosphere before that happens. As the spacecraft approaches the rocket, it uses cameras and sensors to inspect it from a meter away. Study the condition of the rocket, whether it is intact or whether pieces have fallen off and are drifting nearby, and also look for hook points where future spacecraft could be attached.
“Designing a servicer that can climb up and handle three tons of debris presents a lot of challenges,” says Mike Lindsay, Astroscale’s chief technology officer. “The biggest challenge is dealing with uncertainty. The object had been there for 15 years. It's not controlled. We are not communicating with it. So we don’t know how it moves, what it looks like, or how it ages.”